This invention relates to an ultrasonic diagnostic system for observing internal conditions of measured objects by emitting an ultrasonic wave into a measured object, such as a human body and by receiving an acoustic wave based on said ultrasonic wave from inside of the measured object.
An ultrasonic diagnostic system is widely used in order to obtain a tomographic image within a human body. In an ultrasonic diagnostic system, the ultrasonic wave pulse is emitted to the human body, an echo wave, diffracted wave or transmission wave returning from a human body is received and thereby a tomographic image indicating tissue within a human body can be obtained based on the received wave. Namely, such ultrasonic diagnostic system has such a merit of not being invasionable and dangerous, such as an X-ray.
An ultrasonic diagnostic system widely used in general utilizes a pulse echo method of operation. The operation of such an ultrasonic diagnostic system will be explained based on this pulse echo method as an example. The ultrasonic wave pulse of 1 MHz to 10 MHz is emitted to an object from the probe containing the piezoelectric element and converted to an electrical signal. Location information of said mismatching areas can be obtained by displaying the above electrical signal. (This process is called an A-mode). A tomographic image can be obtained by means of sequentially shifting the location or angle of the ultrasonic wave pulse emitting probe in accordance with tomography. Location information of mismatching areas is displayed in accordance with the echo waves of emitted pulses. (This process is called a B-mode.)
In such prior ultrasonic diagnostic systems, the ultrasonic wave pulse emission period is limited to a period which does not elapse until the echo pulse is completely received, since a nest ultrasonic wave pulse is emitted only after a period sufficient for the preceding pulse to return as the echo pulse.
Namely, since a velocity of an ultrasonic wave within a human body is above 1500 m/sec, when a maximum diagnostic depth is considered as L (in meters), the period required for a process from pulse emission to reception of echo is 2L/1500 (sec) and therefor the minimum pulse emission period is 2L/1500 (sec).
Thus, the number of scanning lines obtained in one second is limited to 1500/2L (lines). For example, when L=0.2 m, said number of scanning lines becomes 3750, and it is impossible to obtain sufficient scanning lines in such a case where a tomographic image is required within a very short period of time (0.1 sec) for example.
This restriction on the scanning lines results in a problem when displaying a tomographic image on an ordinary CRT display unit.
Namely, since a frame speed of about 30 frames/sec is necessary for display of movement within a human body without the display flickering. For an ultrasonic device in this application, the number of, the scanning lines of one frame is limited to 25 lines/L, while in a television receiver, when L32 0.2 m, there are 512 scanning lines per frame. This means that a tomographic image is displayed very roughly as compared with the television image, thus curtailing a diagnostic effect.
In other words, the prior ultrasonic diagnostic system cannot obtain the sufficient number of scanning lines for displaying the changes of a dynamic organ such as the heart due to the intrinsic limitation of sound velocity. Consequently the tomographic image of the momentary state of the dynamic organ is displayed very roughly. Thus, the elimination of this disadvantage is especially desired.
The number of scanning lines can be increased very easily by preparing a plurality of probes containing piezoelectric elements and emitting the ultrasonic waves from these probes simultaneously. Such technology is already disclosed in the Japanese published patent application No. 53-96285. However, when the ultrasonic waves of the same frequency are emitted simultaneously from a plurality of probes, it is impossible to distinguish the echo pulses of one probe from the echo pulses of the other probes at the time of reception.
Such a known ultrasonic diagnostic system has a problem that the acoustic waves interfere mutually due to the ultrasonic waves being emitted simultaneously. Thus, in actual operation only an image containing a certain degree of noise can be obtained. Another problem is that a combined display on the screen is difficult to obtain because location detection of the probes is not carried out.